Rehman and colleagues ask that Ibn al-Nefis be recognized as the discoverer of the circulation of the blood. Indeed, he does deserve credit for being the first to describe the lesser or pulmonary circulation (1-3). Keynes (3), however, has pointed out that al-Nefis's realization that the ventricular septum lacked pores and thus that blood must pass through the lungs to get from ventricle to ventricle was theoretical rather than based on animal dissection or experimentation. It remained for Harvey 400 years later to grasp the nature and importance of the systemic circulation, a discovery that served as the birth of modern circulatory physiology.